More effective measures for prevention and treatment of drug abuse and its consequences will not proceed until we increase our understanding of the control drugs have over behavior. It has been well established that drugs can control behavior as discriminative stimuli setting the occasion for the occurrence of specific behaviors and as reinforcing stimuli increasing the probability of drug self-administration. As discriminative stimuli, drugs may actually set the occasion for their own self- administration or the self-administration of other drugs. When one drug serves as a discriminative stimulus for the self- administration of other drugs, self-perpetuating polydrug abuse can be generated: not only its drug-seeking behavior reinforced by a drug's effects, but its effects, functioning as discriminative stimuli, also engender behavior directed at the acquisition of yet other drugs. In this project, aspects of the discriminative stimulus and the reinforcing stimulus functions of drugs will be examined. In particular, animal models will be used to study conditioning factors that may be responsible for relapse, for the abuse of drug combinations, and for the propensity of the abuse of one drug to precede and lead to the abuse of another. Specific drugs that will be studied, sometimes alone and sometimes in combination with one another, include tetrahydrocannabinol (THC), the principal active constituent of marihuana, phencyclidine (PCP), and ethanol.